Express your answer as a mixed number simplified to lowest terms. $20\dfrac{2}{3}-18\dfrac{5}{7} = {?}$
Find a common denominator for the fractions: $= {20\dfrac{14}{21}}-{18\dfrac{15}{21}}$ Convert ${20\dfrac{14}{21}}$ to ${19 + \dfrac{21}{21} + \dfrac{14}{21}}$ So the problem becomes: ${19\dfrac{35}{21}}-{18\dfrac{15}{21}}$ Separate the whole numbers from the fractional parts: $= {19} + {\dfrac{35}{21}} - {18} - {\dfrac{15}{21}}$ Bring the whole numbers together and the fractions together: $= {19} - {18} + {\dfrac{35}{21}} - {\dfrac{15}{21}}$ Subtract the whole numbers: $=1 + {\dfrac{35}{21}} - {\dfrac{15}{21}}$ Subtract the fractions: $= 1+\dfrac{20}{21}$ Combine the whole and fractional parts into a mixed number: $= 1\dfrac{20}{21}$